phy351 ch 6

CHAPTER 6

Metals

PHY351

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METALLIC MATERIAL

Ferrous Metals and alloys

- Metals and alloys that contain a large percentage of iron such as steels and cast irons

Nonferrous metals and alloys

- Metals and alloys that do not contain iron.

- If they do contain iron, it is only in a relatively small percentage.

Metals and Alloys

Ferrous

Eg: Steel,

Cast Iron

Nonferrous

Eg:Copper

Aluminum

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QUESTION 1

1. Define the following materials in terms of their

properties and give an example each of their

application.

a. Stainless steel

b. Brass

c. Cast iron

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PROCESSING OF METAL - CASTING

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Most metals are first melted in a furnace.

Alloying is done if required.

Large ingots are then cast.

Sheets and plates are then produced from ingots by

rolling (wrought alloy products).

Channels and other shapes are produced by extrusion.

Some small parts can be cast as final product.

Example :- Automobile Piston.

Casting Process Casting mold 5

HOT ROLLING OF STEEL

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Hot rolling : Greater reduction of thickneess in a single

pass.

Rolling carried out at above recrystallization

temperature.

Ingots preheated to about 12000C.

Ingots reheated between passes if required.

Usually, series of 4 high rolling mills are used.

COLD ROLLING OF METAL SHEET

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Cold rolling is rolling performed below recrystallization

temperature.

This results in strain hardening.

Hot rolled slabs have to be annealed before cold rolling.

Series of 4 high rolling mills are usually used.

Less reduction of thickness.

Needs high power.

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% Cold work =Initial metal thickness – Final metal thickness

Initial metal thickness

x 100

EXTRUSION

Metal under high pressure is forced through opening in a die.

Common Products are cylindrical bar, hollow

tubes from copper, aluminum etc.

Normally done at high temperature.

Indirect extrusion needs less power however has

limit on load applied

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Container Container

MetalMetal

Die

Direct

Extrusion

Indirect

Extrusion

FORGING

Metal, usually hot, is hammered or pressed into desired shape.

Types:- Open die:

Dies are flat and simple in shape.

(Example products: Steel shafts)

Closed die:

Dies have upper and lower impresion.

(Example products: Automobile engine connection rod)

Forging increases structural properties, removes porosity and increases homogeneity.

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Direct

Forging

Indirect

Forging

DieMetal

DRAWING

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Wire drawing :- Starting rod or wire is drawn through

several drawing dies to reduce diameter.

% cold work =Change in cross-sectional area

Original area

X 100

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Deep drawing:- Used to shape cup like articles

from flats and sheets of metals

MECHANICAL PROPERTIES OF METAL

Stress

Strain

Hardness

Impact Energy

Fracture

Toughness

Fatigue

Creep

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STRESS

Metals undergo deformation under uniaxial tensile force.

Elastic deformation:

Metal returns to its original dimension after tensile

force is removed.

Plastic deformation:

The metal is deformed to such an extent such

that it cannot return to its original dimension

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Engineering stress , σ =

F

A0

Units of Stress are PSI or N/M2 (Pascals)

1 PSI = 6.89 x 103 Pa

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Engineering strain , ε =Change in length

Original length

0

0

Units of strain are in/in or m/m.

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SHEAR STRESS AND SHEAR STRAIN

τ = Shear stress = A

Shear strain γ = Amount of shear displacement

Distance ‘h’ over which shear acts.

S

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Modulus of elasticity (E) or Young’s modulus : Stress

and strain are linearly related in elastic region. (Hooks law)

E = σ (Stress)

ε (Strain)

Stress

Strain

Linear portion of the stress strain curve

Δε

Δσ E =

Δσ

Δε

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Higher the bonding strength, higher is the modulus of

elasticity.

Examples:

Modulus of Elasticity of steel is 207 Gpa.

Modulus of elasticity of Aluminum is 76 Gpa

YIELD STRENGTH

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Yield strength is strength at which metal or alloy show significant amount of plastic deformation.

0.2% offset yield strength is that strength at which 0.2% plastic deformation takes place.

Construction line, starting at 0.2% strain and parallel to elastic region is drawn to 0.2% offset yield strength.

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ULTIMATE TENSILE STRENGTH

Ultimate tensile strength (UTS) is the maximum strength reached by the engineering stress strain curve.

Necking starts after UTS is reached.

Al 2024-Annealed

Al 2024-Tempered

S

T

R

E

S

S

Mpa

Strain

Necking Point

Stress strain curves of

Al 2024 With two different

heat treatments. Ductile

annealed sample necks more

More ductile the metal is, more is the necking before failure.

Stress increases till failure. Drop in stress strain curve is due to stress calculation based on original area.

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PERCENT ELONGATION

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Percent elongation is a measure of ductility of a material.

It is the elongation of the metal before fracture expressed

as percentage of original length.

% Elongation = Final length* – initial Length*

Initial Length

PERCENT REDUCTION IN AREA

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Percent reduction area is also a measure of ductility.

The diameter of fractured end of specimen is measured

using caliper.

% Reduction

Area=

Initial area – Final area

Initial area

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Percent reduction in area in metals decreases in case of

presence of porosity.

TRUE STRESS –TRUE STRAIN

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True stress and true strain are based upon instantaneous

cross-sectional area and length.

True stress is always greater than engineering stress.

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True Stress = σt =

True Strain = εt =

F

Ai (instantaneous area)

i

i

A

ALn

l

lLn

di

0

00

QUESTION 2

1. A 0.5cm diameter aluminium bar is subjected to a force of 500N.

Calculate the engineering stress in MPa on the bar.

(Answer: 25.5 MPa)

2. A 1.25cm diameter bar is subjected to a load of 2500 kg. Calculate

the engineering stress on the bar in MPa.

(Answer: 200 MPa)

3. A sample of commercially pure aluminium 1.27cm wide, 0.1cm

thick and 20.3cm long that has gage markings 5.1cm apart in the

middle of the sample is strained so that the gage markings are

6.7cm apart. Calculate the engineering strain and the percent

engineering strain elongation that the sample undergoes.

(Answer: 0.31, 31%)30

4. A 12.7mm diameter round sample of a 1030 carbon steel is pulled

to failure in a tensile testing machine. The diameter of the sample

was 8.7mm at the fracture surface. Calculate the percent reduction

in area of the sample.

(Answer: 53%)

5. A 70% Cu-30% Zn brass sheet is 0.12cm thick and is cold-rolled

with a 20 percent reduction in thickness. What must be the final

thickness of the sheet?

(Answer: 0.096cm)

6. Calculate the percent cold reduction when an aluminium wire is

cold-drawn from a diameter of 6.5mm to a diameter of 4.25mm.

(Answer: 57.2%)

7. A tensile specimen of cartridge brass sheet has a cross section of

10 mm x 4mm and a gage length of 51mm. Calculate the

engineering strain that occurred during a test if the distance

between gage markings is 63 mm after the test.

(Answer: 0.235)

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8. Compare the engineering stress and strain with the true test and

strain for the tensile test of low-carbon steel that has the following

test values.

Load applied to specimen = 69 000N

Initial specimen diameter = 1.27 cm

Diameter specimen under 69 200 N load = 1.2 cm

(Answer: 544.6 MPa, 610 Mpa, 0.12, 0.117)

9. A 20cm long rod with a diameter of 0.25cm is loaded on an FCC

single crystal. Calculate

a. The engineering stress and strain at this load

(Answer: 1019 MPa, 0.147)

b. The trues tress and strain at this load.

(Answer: 1443 MPa, 0.349)

HARDNESS

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Hardness is a measure of the resistance of a metal to

permanent (plastic) deformation.

General procedure:

Press the indenter that

is harder than the metal

Into metal surface.

Withdraw the indenter

Measure hardness by

measuring depth or

width of indentation.

34Rockwell hardness tester

FRACTURE

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Fracture results in separation of stressed solid into two or more parts.

There are two types of fracture:

Ductile fracture

Brittle Fracture

DUCTILE FRACTURE

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Ductile fracture :

High plastic deformation & slow crack propagation (fracture due to slow crack propagation).

Three steps :

Specimen forms neck and cavities within neck.

Cavities form crack and crack propagates towards surface, perpendicular to stress.

Direction of crack changes to 450 resulting in cup-cone fracture.

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Ductile fracture

BRITTLE FRACTURE

No significant plastic deformation before fracture (fracture

due to rapid crack propagation).

Common at high strain rates and low temperature.

Three stages:

Plastic deformation concentrates dislocation along slip

planes.

Microcracks nucleate due to shear stress where

dislocations are blocked.

Crack propagates to fracture. 38

Example:

HCP Zinc ingle crystal under high stress along {0001}

plane undergoes brittle fracture.

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SEM of ductile fracture SEM of brittle fracture

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Brittle Fracture

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Brittle fractures are due to defects like

Folds

Undesirable grain flow

Porosity

Tears and Cracks

Corrosion damage

Embrittlement due to atomic hydrogen

At low operating temperature, ductile to brittle transition

takes place

TOUGHNESS

Toughness is a measure of energy absorbed before

failure.

Impact test measures the ability of metal to absorb

impact.

Toughness is measured using impact testing machine

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FRACTURE TOUGHNESS

Cracks and flaws cause stress concentration.

where

K1 = Stress intensity factor.

σ = Applied stress.

a = edge crack length

Y = geometric constant.

aYK 1

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KIc = critical value of stress intensity factor (fracture

toughness)

Example:

Al 2024 T851 26.2MPam1/2

4340 alloy steel 60.4MPam1/2

QUESTION 3

1. A structural plate component of an engineering design must

support 207 MPa in tension. If aluminium alloy 2024-T851 is

used for this application, what is the largest internal flaw size

that this material can support? (Use Y = 1, KIc =26.4 MPa)

(Answer: 5.18 mm)

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FATIGUE

The phenomenon leading to fracture under repealed

stresses having a maximum value less than the ultimate

strength of the material.

Metals often fail at much lower stress at cyclic loading

compared to static loading.

Crack nucleates at region of stress concentration and

propagates due to cyclic loading.

Failure occurs when cross sectional area of the metal too

small to withstand applied load.

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Fatigue fractured surface of keyed shaft

Fracture started here

Final rupture

Factors affecting fatigue strength:

Stress concentration: Fatigue strength is reduced by stress

concentration.

Surface roughness: Smoother surface increases the fatigue

strength.

Surface condition: Surface treatments like carburizing and

nitriding increases fatigue life.

Environment: Chemically reactive environment, which

might result in corrosion, decreases fatigue life.

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CREEP

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Creep is a time-dependent plastic deformation when subjected to a constant stress or load.

Important in high temperature applications:

i. Primary creep: creep rate decreases with time due to strain hardening.

ii. Secondary creep: Creep rate is constant due to simultaneous strain hard- ening and recovery process.

iii. Tertiary creep: Creep rate increases with time leading to necking and fracture.

50Creep curve.

The slope of the linear part of the curve is the steady-state creep rate.

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Creep test determines the effect of temperature and

stress on creep rate.

Metals are tested at constant stress at different

temperature & constant temperature with different stress.

High temperature

or stress

Medium temperature

or stress

Low temperature

or stress

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Creep strength: Stress to produce minimum creep rate

of 10-5%/h at a given temperature.

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Creep rupture test is same as creep test but aimed at failing the specimen.

Plotted as log stress versus log rupture time.

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Time for stress rupture decreases with increased stress

and temperature

REFERENCES

A.G. Guy (1972) Introduction to Material Science,

McGraw Hill.

J.F. Shackelford (2000). Introduction to Material Science

for Engineers, (5th Edition), Prentice Hall.

W.F. Smith (1996). Principle to Material Science and

Engineering, (3rd Edition), McGraw Hill.

W.D. Callister Jr. (1997) Material Science and

Engineering: An Introduction, (4th Edition) John Wiley.

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